The ux of Slack is essentially screen+irc implemented in JS with emotes. It enabled technical and non-technical people to use the same tool. The key to success is not technical, it's that they tailored the product to a specific group that would then lock itself in.
I didn't understand Discord's success, but comments here point that gamers couldn't find free group-voice apps at a critical time. Here again, they tailored the product to a group that would then voluntarily lock itself in.
Later, they sell the companies with valuations based on the captured user bases.
Another big thing that the current crop of winners has going for it is that cloud hosting allows applications to launch literally for free and scale quite a bit without paying much of anything in infrastructure costs. That also wasn't an option 10-20 years ago.
Discord's a pretty good product, and they've got the engineers and money to get better, but the only reason they won is because of timing. Same for Slack; there were identical products to Slack that tried for decades to gain traction, but they weren't free, because that business model didn't exist at the time.